1. Field of the Invention
The present invention relates to a negatively chargeable toner for developing latent electrostatic images, and more particularly to a negatively chargeable toner comprising a binder resin, a coloring agent and a negative charge controlling agent.
2. Discussion of Background
For developing latent electrostatic images to visible toner images with a dry-type developer (toner), either a one-component dry-type developer or two-component dry-type developer is conventionally employed as disclosed in Japanese Laid-Open Patent Application 61-147261.
The one-component dry-type developer comprises a toner, while the two-component dry-type developer comprises a mixture of a toner and carrier particles.
When the two-component dry-type developer is used for developing latent electrostatic images, the toner and the carrier are triboelectrically charged to opposite polarities by mixing them, and the charged toner is electrostatically attracted to the latent electrostatic images provided with a polarity opposite to that of the toner, so that the latent electrostatic images are developed to visible toner images.
For use with such a two-component type developer, several development methods are known depending on the kind of toner and carrier. For example, a magnetic brush development method of using a carrier of iron powder, a cascade development method of using a carrier of beads, and a fur brush development method are known.
In the case of the one-component dry-type developer, a powder cloud development of spraying toner particles in the air, a contact development (or touch-down development method) of bringing toner particles into contact with latent electrostatic images and an induction development method of bringing an electroconductive magnetic toner into contact with latent electrostatic images are used.
Toners for use in the above-mentioned various development methods are usually prepared by dispersing a coloring agent such as carbon black in a binder resin comprising a natural resin or synthetic resin and crushing the mixture. For example, a coloring agent is dispersed in a binder resin such as polystyrene and the mixture is pulverized until the particle diameter reaches about 1 to 30 .mu.m. Moreover, a magnetic toner can be obtained by adding a magnetic material such as magnetite to the above-prepared toner.
The toners for use in the previously mentioned development methods are negatively or positively charged according to the polarity of the latent electrostatic images to be developed. In order to charge the toner particles, the triboelectric charging property of a resin component contained in the toner can be utilized. However, the desired chargeability cannot be always obtained by triboelectric charging of the resin component, so that fogging easily takes place in the obtained images, which makes the images unclear. Therefore, to impart the desired triboelectric chargeability to the toner, dyes or pigments which can impart such triboelectric chargeability to the toner or charge controlling agents are generally added to the toner.
Examples of the conventional charge controlling agents which apply a negative charge to the toner include metal complex salts of a monoazo dye, nitrohumic acid and salts thereof, Co, Cr and Fe complexes of salicylic acid, naphthoic acid and dicarboxylic acid, sulfonated copper phthalocyanine pigments, nitro group- or halogen-introduced styrene oligomers, chlorinated paraffin and melamime resin.
The structure of the above-mentioned charge controlling agents is complicated and the characteristics thereof are unstable. In addition, the above-mentioned charge controlling agents tend to be decomposed or changed by mechanical shocks, friction and changes in the ambient temperature and humidity when they are kneaded with other components under application of heat to prepare a toner. With many of the above charge controlling agents, the chargeability is susceptible to the changes in the environmental conditions.
Moreover, when the toner containing such a conventional charge controlling agent is used for an extended period of time, the toner particles are apt to be deposited on the surface of a photoconductor due to the improper charging thereof. As a result, a so-called toner-filming phenomenon takes place.
Recently, polyester resin and epoxy resin are usually employed as a binder resin for the toner because they do not adhere to a PVC film and not impair the original color of the toner, particularly in the case of a color toner.
However, when the polyester resin and epoxy resin are used as the binder resins, the charge quantity of the toner is insufficient or the charge quantity decreases while in use even if high charge quantity is initially obtained. This causes the fogging of images or scattering of the toner on a copy paper. This is because the functional groups contained in the polyester resin and epoxy resin, such as --COOH group and --OH group, hinder the electric charge from remaining on the toner particles.
In addition to the above, toners comprising a fluorine-containing compound have been proposed, for instance, in Japanese Laid-Open Patent Applications 53-124428, 55-5025, and 58-66950. More specifically, in Japanese Laid-Open Patent Application 53-124428, a particular aliphatic fluorocarbon or fluorochlorocarbon is employed, in Japanese Laid-Open Patent Application 55-50251, particular fluorine-containing compounds are employed, and in Japanese Laid-Open Patent Application 58-66950, a capsule-type toner which is surface-treated by an organic fluoro compound is proposed. However, these toners are not always satisfactory for use in practice.